How to make a PWA from an old React site?

Question

React

Please tell me, if there is an old spa site on React.js (maybe the old create-react-app or my own build was used, I don’t know for sure), how to turn it into PWA so that you can create a shortcut and make a push notification? So far, it only comes to mind to generate a new application (npx create-react-app my-app --template cra-template-pwa) and transfer the functionality there, but it will take a long time and probably break a lot, maybe there is a more subtle way?